Routers in a network forward packets from one to another to deliver them to destination computers. The routers exchange their routing information with each other to recognize the right transmission path to use for delivery of packets to their destinations. Routing information sent from a router indicates transmission paths that the sending router recognizes. Each router uses such routing information from other routers to make forwarding decisions according to specified destinations of packets.
When there are two or more transmission paths to the destination computer, the router chooses an appropriate path by using a path selection algorithm defined in routing protocols, such as Open Shortest Path First (OSPF) and Routing Information Protocol (RIP). These routing protocols are basically designed to take the shortest path to reach the destination. Some existing sophisticated control techniques based on such routing protocols select shortest paths while minimizing the peak usage of the network.
The above-described existing routing protocols assume that the data traffic (e.g., packets) in a network are transported along a predetermined path. For this reason, the routers on such predetermined paths are supposed to be always active even if the total traffic may become low in some time period.
On the other hand, the power consumption of network components has increased with their performance enhancement, and overall power management of a network has become an increasingly important part of operations of the network system. For example, one proposed technique achieves this by automatically detecting a data highway experiencing a reduction in the amount of data traffic and making the traffic detour around the detected highway via another highway. This detouring of data traffic enables some devices in a network node to go into an inactive state, and the power consumption can thus be reduced by shutting those inactive devices down. Another proposed technique is to consolidate lightly-loaded paths into one, so that more routers can become inactive.
As a different approach to the noted requirement, other proposed techniques distribute the traffic when the network encounters congestion. For example, the traffic received by an input edge router is distributed to a plurality of paths established between the input edge router and an output edge router.
The following literature describes conventional routing techniques related to this application:                Japanese Laid-open Patent Publication No. 2001-119730        Japanese Laid-open Patent Publication No. 2004-48330        Akiko Yamada et al., “A Study for Green Network (3)—ECO Routing—,” Proceedings of the IEICE General Conference, The Institute of Electronics, Information and Communication Engineers (IEICE), Mar. 4, 2009, 2009_Communications (2), pp. 252.        
Congestion of traffic may occur in some local part of a network, rather than in the entire network. For example, the traffic may be congested in some particular links coupling routers. The congested links may be part of a transmission path of data traffic from its input edge router to its output edge router. One method to solve this link congestion is to reconfigure the network so that the traffic on the current path will be distributed to other alternative paths.
However, distributing the traffic of near congested links may not always be an optimal solution in terms of increase of power consumption. For example, the network may include some routers which have been operating in a power saving mode (or sleep mode) as a result of the aforementioned consolidation of transmission paths. Distributing the entire traffic of such links would awake those sleeping routers and thus invite an increase of power consumption in the network. Suppose, for example, that a link is experiencing a traffic load that exceeds a specified threshold, and there are a number of paths running through that link. In this case, it may be possible to avoid congestion by only partly distributing the load of those paths. However, distributing the traffic load of all those paths to other paths would activate the sleeping routers more than necessary and thus lead to an unnecessary increase of power consumption.
It is noted that the above-described difficulties in congestion control may also apply to, for example, the case of searching for alternative routes to detour the traffic around a failed link.